In a liquid crystal display device, a classification based on an operating mode for liquid crystals includes modes of PC (phase change), TN (twisted nematic), STN (super twisted nematic), ECB (electrically controlled birefringence), OCB (optically compensated bend), IPS in-plane switching), VA (vertical alignment) and PSA (polymer sustained alignment). A classification based on a driving mode in the device includes PM (passive matrix) and AM (active matrix). The PM is further classified into static, multiplex and so forth, and the AM is classified into TFT (thin film transistor), MIM (metal-insulator-metal) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to the production process. A classification based on a light source includes a reflection type utilizing natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both natural light and a backlight.
These devices contain a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to give an AM device having good general characteristics. Table 1 below summarizes the relationship between the general characteristics of the two. The general characteristics of the composition will be further explained on the basis of a commercially available AM device. The temperature range of a nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is 70° C. or higher and a desirable minimum temperature of the nematic phase is −10° C. or lower. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying moving images on the device. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1General Characteristics of Composition and AM DeviceGeneral CharacteristicsGeneral CharacteristicsNo.of Compositionof AM Device1wide temperature range ofwide usable temperature rangea nematic phase2small viscosity 1)short response time3suitable optical anisotropylarge contrast ratio4large positive or negativelow threshold voltage and smalldielectric anisotropyelectric power consumptionlarge contrast ratio5large specific resistancelarge voltage holding ratio andlarge contrast ratio6high stability to ultravioletlong service lifelight and heat1) A liquid crystal composition can be injected into a liquid crystal cell in a shorter period of time.
The optical anisotropy of the composition relates to the contrast ratio of the device. The product (Δn×d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on the kind of operating mode. In a device having a TN mode, a suitable value is approximately 0.45 micrometer. In this case, a composition having a large optical anisotropy is desirable for a device having a small cell gap. A large dielectric anisotropy of the composition contributes to a low threshold voltage, small electric power consumption and a large contrast ratio of the device. Accordingly, a large dielectric anisotropy is desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, a composition having a large specific resistance is desirable at room temperature and also at a temperature close to the maximum temperature of a nematic phase in the initial stage. A composition having a large specific resistance is desirable at room temperature and also at a temperature close to the maximum temperature of a nematic phase after it has been used for a long time. The stability of the composition to ultraviolet light and heat relates to the service life of the liquid crystal display device. In the case where the stability is high, the device has a long service life. These characteristics are desirable for an AM device used in a liquid crystal projector, a liquid crystal television and so forth.
In an AM device having a TN mode, an optically active compound is mixed with the composition for the purpose of inducing a helical structure and giving a twist angle in liquid crystals, and a composition having positive dielectric anisotropy is used. On the other hand, a composition having negative dielectric anisotropy is used for an AM device having a VA mode. A composition having positive or negative dielectric anisotropy is used for an AM device having an IPS mode. A composition having positive or negative dielectric anisotropy is used for an AM device having a PSA mode. Examples of liquid crystal compositions having positive dielectric anisotropy are disclosed in the following patent documents.